(Topic ID: 236285)

Trying to fix WPC A-12738 sound board (T2)

By pintibo

5 years ago


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replaced more caps (resized).JPG
burnt C47 (resized).JPG

#1 5 years ago

For some time now I've been trying to restore the sound quality in the T2 that I recently picked up.
The problem is that some music is distorted. I document it here in the hope that someone might have a good suggestion and to be of use for people with similar problems.

In this video you can hear how it sounds (note especially 03. Multiball Lit Tune):

The first obvious problem was that C47 burned out at some point (see photo).
After replacing it (together with C46, C26 and C28) and reseating connectors and ICs the problem remained.

I'm really grateful for all the useful information about pre-dcs audio boards on Pinside, which has helped me understand more and more of the workings of it.
I'll give some more info in the following posts.

burnt C47 (resized).JPGburnt C47 (resized).JPG

#2 5 years ago

I am very happy I have a TZ with a working A-12738 sound board that I can use to swap out parts.

I swapped out the sound ROMs U14, U15 and U18 between the two boards and installed the working board in the T2. After the swap all T2 sounds are normal. This rules out any issues with the ROMs, speakers and wiring.

Next, I also put the bad audio board in the TZ, just for the sake of it and because I was interested if the TZ sounds would give me any clues.


From the video it is clear voice is unaffected and the problem is only apparent during music playback.

My next action was to replace more caps. I swapped out:
4700 uf: C24, C25
47 uf: C15, C36, C38

I ordered a LM1875 and replaced this while I had the board out.
I could hear no difference, so I doubted I was going in the right direction.

replaced more caps (resized).JPGreplaced more caps (resized).JPG
#3 5 years ago

I don't really like the idea of just swapping parts without understanding their influence on the problem. Of course it's especially unsatisfying when you can't even notice a difference after doing it.

I decided to do some reading up on the working of the sound board. Altan gives useful insight about the channels in his posts and on his website (https://www.aaarpinball.com/TwilightZone/TwilightZone.htm).

So I quickly found out there are 4 channels on the board:
CVSD (voice samples)
CH1, CH2 (both music)
DAC (not really sure at first, but I found out later this at least is responsible for percussion)

I read somewhere the music on the CH1 and CH2 is generated by two Yamaha chips (YM2151 and YM3012). At first I was kind of annoyed with myself for not sooner swapping out these socketed ICs between boards, because this would be an easy check.
After swapping them out however, there was no audible change in audio quality.

I decided to dig deeper and wanted to see if I could analyse the signals on the channels separately. I attached my oscilloscope and probed:
- U7 at pin 7 for CVSD
- U7 at pin 8 for DAC
- U8 at pin 8 for CH1
- U8 at pin 14 for CH2

With the oscilloscope I could confirm that CVSD was responsible for speech and CH1 and CH2 for music. It also showed the DAC is involved in music playback as well.
I managed to export some signals from my scope to my computer in csv-format and imported them in the free software Audacity.
It made it possible for me to play back short signals and compare two simultaneously sampled channels. This was pretty awesome!

I found out CH1 and CH2 actually sound equal and excellent. I really like the sound of these Yamaha chips!
In the screenshot you can see both channels are indistinguishable. I wonder why two channels are used. Does anybody have an idea?

After comparing the signals of CH1 and DAC I found some erratic signals on the DAC channel, that I show in the following clip.

I made the following observations:
- CH1 is good
- The DAC signal is very asymmetrical, which looks strange to me. Is this due to the place where I tapped the signal or is it part of the problem?
- The DAC signal seems to collapse at certain moments that correspond with audible cracks.

After this I decided to replace some more capacitors (47uf: C32, C35). C35 seems to be closely linked to the DAC circuit.

But to no resolve.

At the moment I am convinced the problem is in or around the DAC (AD7524). I am not ready to replace the chip yet, I still have some questions:
- Is the AD7524 blown or something connected to it?
- Is there a relationship with the burnt-out C47?
- I would like to look at the DAC signal in my good board. Is this signal symmetrical?

Any feedback is highly appreciated!

#4 5 years ago

Sorry. Can't help but a very interesting read.

#5 5 years ago

Like the above poster stated, you’ve done more to uncover the inner workings of this board than most people have. Good luck with the repair.

#6 5 years ago

Thanks guys!

Managed to do one more test. I simultaneously captured the signal on CH1 and DAC on my TZ. The DAC on this board also showed the asymmetric signal (DC component). I guess this is normal.

The percussion was clean, as expected. I've been looking at the board schematics again. I'm not much of an electro-technician, but I see the DAC signal is run through a TL082 op-amp.

Does anybody know if there is an easy way to test these? I guess I'll just order a new one anyway, since they are cheap.

#7 5 years ago

I think you're on the right track with the audio isolation of the DAC. Have you tried checking the pinouts of the chip for resistance? The data sheet for it has a reference input of 5k ohm min to 20k max (Pin 15 to ground)
Edit again: ok had to make sense of this for myself; if you go from pin 15 on the chip to the leg of R51 that is closest to the chip you should get 10,560 ohms and if you repeat that on R50 you should get 5,260 ohm (Or I'm not understanding how R51 and R50 are actually parallel which gives me 3,760 ohm)

Edit: the gain reference resistor would be good to check pin 16 to 1, should be 10k ohm

https://www.analog.com/media/en/technical-documentation/data-sheets/ad7524.pdf

#8 5 years ago
Quoted from Asmig:

I think you're on the right track with the audio isolation of the DAC. Have you tried checking the pinouts of the chip for resistance? The data sheet for it has a reference input of 5k ohm min to 20k max (Pin 15 to ground)
Edit again: ok had to make sense of this for myself; if you go from pin 15 on the chip to the leg of R51 that is closest to the chip you should get 10,560 ohms and if you repeat that on R50 you should get 5,260 ohm (Or I'm not understanding how R51 and R50 are actually parallel which gives me 3,760 ohm)
Edit: the gain reference resistor would be good to check pin 16 to 1, should be 10k ohm
https://www.analog.com/media/en/technical-documentation/data-sheets/ad7524.pdf

Thanks for taking the time to think about this! I am not really sure what you are suggesting. Do you mean for me to check the resistances connected to the DAC? You don't mean internal resistance of the DAC, do you? I expect my new op-amp to arrive tomorrow, before replacing that I'll first measure the resistors!

#9 5 years ago
Quoted from pintibo:

Thanks for taking the time to think about this! I am not really sure what you are suggesting. Do you mean for me to check the resistances connected to the DAC? You don't mean internal resistance of the DAC, do you? I expect my new op-amp to arrive tomorrow, before replacing that I'll first measure the resistors!

Take a look at the input resistance to the DAC, the resistors that go to it (R50, R51, R38) look a little discolored to me. Not sure how they could even be damaged unless the +12 went too high. You could also check and replace C34 if you haven't already.

Edit: you have a few other resistors that look discolored, (R21, R22, R23) that would be good to check and compare to your good working board

#10 5 years ago
Quoted from Asmig:

Take a look at the input resistance to the DAC, the resistors that go to it (R50, R51, R38) look a little discolored to me.

they check out ok. I actually didn't notice the discoloration before. Thanks for pointing that out. I think the discoloration is caused by C47 that went up in smoke and is located right beneath them.

Quoted from Asmig:

You could also check and replace C34 if you haven't already.

I haven't. On the schematic it looks close to the DAC, but it's actually only related to CVSD. To me it doesn't seem likely to be the cause of the problems.

Quoted from Asmig:

Edit: you have a few other resistors that look discolored, (R21, R22, R23) that would be good to check and compare to your good working board

I roughly checked them. They should be different from my TZ board. Correct me if I'm wrong, but I think these only influence the balance of the channels.

While I had the board out I also replaced the TL082 (U32). Still no luck in improving the sound quality. It sounds exactly the same.

I'll call it a night for now. Hope I wake up with some genious idea.

#11 5 years ago

I would go ahead and finish replacing any electrolytic caps just to have to worry about them. Looking at your picture again it appears C12 might be damaged, like a chip has been broken off of it, might be worth double checking.

1 week later
#12 5 years ago

Hi, did you ever get to the bottom of this issue?

#13 5 years ago

Thanks for showing interest in my little project. I didn't have time previous week, but hope tomorrow I can do some more tests. To be honest, I was a little bit frustrated after my first promising steps didn't get a follow-up.

1 month later
#14 4 years ago

SO any news? great read! Hope you get her figured. Having some sound woes myself right now.
Cheers

#15 4 years ago

from the sound of that. First thing I would do is replace every elec. cap on that board along with the tants 1uf and 10 uf with 35 or 50 v parts and see from there. AMAZING what replacing all of those does for sound.

Also check for the heat transfer paste on the AMP chip or it will get hot and sound just like this. And check for dc voltage at the speakers. there should be none. If there is the caps are bad feeding the LM1875 . Caps C46-C47 (1 mfd Tant), C20 (10 mfd tant), C22 (22 mfd), C23 (.22 mfd)

4 months later
#16 4 years ago

I have the exact issue on a Dr. Who board. I'm using WhiteWater ROMs during the debug.
I've isolated the issue to noise coming from the DAC sound lane.
The TL082 and AD7524 have been replaced with known working parts. Still no joy.

I have another theory...I'll post back.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info

#17 4 years ago

I have a half dead T2 board as well but just missing a bunch of speech callouts. Actually this is a Dr Who board that was put into a T2. I also had to replace the c46/c47 combo when the tant cap did a fireworks display. I lost speech a couple years after the repairs so I doubt they're related. I dont think it would be a bad idea to recap the whole thing and maybe even look at the resistors for drifting on this near 30 year old board. Fascinating thread tho with the in depth tests. You've certainly advanced the knowledge base with your findings

#18 4 years ago
Quoted from WW2GURU:

I have a half dead T2 board as well but just missing a bunch of speech callouts. Actually this is a Dr Who board that was put into a T2. I also had to replace the c46/c47 combo when the tant cap did a fireworks display. I lost speech a couple years after the repairs so I doubt they're related. I dont think it would be a bad idea to recap the whole thing and maybe even look at the resistors for drifting on this near 30 year old board. Fascinating thread tho with the in depth tests. You've certainly advanced the knowledge base with your findings

Replacing all the caps and the SRAM usually brings them back to life.

Sometimes replacing U21 and U28 as a pair works as reported by Ken Layton awhile back.

5 months later
#19 4 years ago

We had the exact same problem, and after changing some components that was bad the problem was still there.
Anyway, all the speakers what shorted, so if you ohm measure on the speakers that should be ~4 ohms, they where only 1-1.5 ohms. So we changed all the speakers and the problem was gone.
If the amp chip gets hot >60C you probably have the same problem.

Hope it helps. Yeah I know the answer should have come 5 months ago...

#20 4 years ago
Quoted from GeekJoan:

Yeah I know the answer should have come 5 months ago...

Still not too late... I never was able to fix the audio. I managed to play the hell out of the T2. I really love the game and would enjoy fixing the sound eventually.
Thanks for the suggestion. I will check the resistance of the speakers. I think I already checked it, but that must have been a year ago.

Your name sounded familiar, so I looked it up. I watched all your videos on the love you put into your TZ. Nice!

2 years later
#21 1 year ago

I am reviving this thread in hopes that there was a solution found.

I have an Addams Family with the very same issue, distortion in the music and not the speech call outs.

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